1. Field of the Invention
This invention pertains to optical transmission in a barium-gallium-germanium oxide (BGG) glass material and to a method for making the BGG glass material.
2. Description of Related Art
There are numerous sensor and laser systems operating in the visible to infrared wavelength band of about 0.5 to 5 microns (μm). These systems require windows for protection. The size, shape and desired properties of the window depend on the application. These applications can be very military specific, such as sensor windows on aircraft, reconnaissance windows on military aircraft and un-attached vehicles, and missile domes, to very large windows for high energy laser systems. On the other hand, the applications can be very commercial, such as small inexpensive lenses for infrared optics and windows for all visible and mid-wave lasers. Materials choice for any application is strongly dependent on the required properties, desired size and shape and material availability. For example, applications such as high energy laser systems, require large windows in 20″ to 75″ diameter sizes with very low absorption coefficient, and a negative or very low positive dn/dT value that can minimize the optical path distortion. For some sensor applications, hemispherical or hyper hemispherical dome-shaped window may be required which are difficult and expensive to shape using currently used window materials. For some reconnaissance applications, some systems require large, i.e., larger than 13″ diameter windows which currently limits the choice of the material to Cleartran which is a zinc sulfide (ZnS) material obtained by CVD followed by hot isostatic pressing. For lenses and other infrared optics, the material of choice is currently Cleartran since there is no low-cost alternate material.
Present commercially available window materials for the visible-infrared wavelength region are either too soft or have limited transmission and all are too expensive. Sapphire has excellent mechanical properties and can withstand the adverse environmental conditions but its transmission at 5.0 μm is limited by multi-phonon absorption. Sapphire windows are also very expensive and are difficult to polish in complex shapes. The cost of polishing sapphire is about five times higher than polishing glass. In addition, sapphire windows are only available in sizes of about 10″ in diameter coming from a 13″ diameter boule. Attempts to fabricate larger size sapphire have been unsuccessful due to crystal growth problems since sapphire is a single-crystal material and its growth is not only slow but it is also problematic. For some systems requiring larger size windows, Cleartran material has been the material of choice by default. Cleartran material has good transmission in the mid-infrared wavelength region but suffers from scattering in the visible region due to the presence of grain boundaries. Cleartran material is also too soft which results in poor rain and sand erosion resistance of the windows and additionally raises the cost of polishing. The polishing cost for Cleartran is almost four times higher than polishing glass. Cleartran also has a high dn/dT which results in defocusing or loss of power due to large optical path distortion for high energy laser applications. Cleartran material (ZnS) also has a high nonlinear coefficient which causes the window to emit visible green light during transmission of high intensity (1.06 μm) laser light through the material. Infrared lenses and windows for laser cavities for commercial applications are expensive due to the cost of re-finishing the surfaces.
The material disclosed in the earlier U.S. Pat. No. 5,305,414 on BGG glass exhibited limited glass stability and infrared transparency due to water impurity (OH−), see FIG. 1. This poor stability led to crystallization when attempts were made to make windows large enough for practical applications, i.e., larger than 4 inches in diameter. Crystallization gave high optical scatter while the OH absorption bands considerably reduced the optical transmission in the mid-infrared range due to extrinsic absorption. The OH− concentration in the prior art BGG glass was greater than 10 ppm. Consequently, the glass as disclosed previously, could not and was not used for practical applications, despite its great potential.